The present invention relates to a sintered stainless steel exhibiting markedly improved resistance to stress corrosion cracking and the production thereof, the steel comprising a matrix phase of a substantially ferritic structure and a dispersing phase containing an austenitic area. The dispersing phase is selected from the group consisting of a single austenitic structure, an austenitic+ferritic structure, an austenitic+martensitic structure, and an austenitic+ferritic+martensitic structure.
As is well known in the art, stainless steel is classified into martensitic, ferritic, austenitic, and duplex types.
Ferritic stainless steel is not expensive and it exhibits good resistance to stress corrosion cracking. However, it has poor toughness and its weldability is not good.
Austenitic stainless steel exhibits good toughness as well as extremely high resistance to corrosion under usual conditions. However, in general, it is expensive since it contains a relatively large amount of Ni and it does not exhibit good resistance to stress corrosion cracking ("SCC"hereunder). The incorporation of a relatively large amount of Ni is effective for improving the resistance to SCC to some extent, but the effect derived from the addition of nickel saturates at a certain level. Furthermore, the addition of nickel makes the steel expensive, resulting in limited applications therefor.
Duplex stainless steel has been proposed so as to eliminate the above-mentioned shortcomings, and it has not only the advantages which the ferritic stainless steels have but also those of the austenitic stainless steels. Duplex steel also exhibits the same level of toughness as austenitic stainless steel does and much better SCC resistance.
Regarding the SCC resistance of duplex stainless steel, an article by Edeleanu appeared in the Journal Iron Steel Inst., 173, 140 (1953) describing the influence of the amount of .delta.-ferritic phase in 18Cr-8Ni-Ti steels on SCC resistance. Since then, a number of other articles have been published. It has been reported that alloying elements, heat treatment conditions, and the amount of ferritic phase have an influence on SCC resistance.
As is well known in the art, duplex stainless steel is characterized by a high ultimate stress against SCC. FIG. 1 and FIG. 2 are graphs disclosed in the Journal of Corrosion Engineering, Vol. 30, No. 4, pp. 218-226 (1981) by one of the inventors of the present invention. FIG. 1 shows the SCC resistance in a 427K, 45% MgCl.sub.2 solution for 25Cr stainless steel test samples of which the nickel content was varied. The test samples were dipped in a boiling solution for 2000 hours. The ordinate is a stress ratio, i.e., the ratio of the ultimate stress against the SCC resistance to the 0.2% yielding point (.sigma..sub.th /.sigma..sub.0.2). The higher the ratio, the better is the SCC resistance. Cracking does not occur for Ni-free ferritic stainless steel, but the ratio of .sigma..sub.th /.sigma..sub.0.2 rapidly decreases for a ferritic stainless steel which contains a very small amount of Ni. The ratio .sigma..sub.th /.sigma..sub.0.2 reaches a minimum when the nickel content is 2%. For 6-8% Ni steels, the value of .sigma..sub.th /.sigma..sub.0.2 increases for the reason that the structure comprises a duplex of a ferritic structure and an austenitic one. However, the SCC resistance of the duplex stainless steel is still inferior to a Ni-free ferritic stainless steel. It is thought that this is because the ferritic phase of the duplex stainless steel contains a relatively large amount of Ni due to redistribution between ferritic phase and austenitic phase.
FIG. 2 shows graphs of the SCC resistance determined for 25Cr-6Ni duplex stainless steel (designated by the symbol "0"), 28Cr-4Ni ferritic stainless steel, the composition of which corresponds to that of the ferritic phase of the duplex steel (designated by the symbol " "), and 21Cr-9Ni austenitic stainless steel, the composition of which corresponds to that of the austenitic phase of the duplex steel (designated by the symbol ".DELTA.". The corrosion tests were carried out under the same conditions as those used in the case of FIG. 1. Graph (a) shows the relationship between the applied stress and the time to failure. Graph (b) shows the stress ratio (.sigma..sub.th /.sigma..sub.0.2) plotted against the time to failure.
A 28Cr-4Ni ferritic stainless steel, though it is designated as ferritic one, exhibits less SCC resistance since it contains 4% of Ni. It is supposed that this is the reason why SCC propagates through a ferritic phase, detours an isolated austenitic phase, and stops open reaching another austenitic phase in conventional duplex stainless steels.
FIG. 3 schematically illustrates the above-described mechanism of SCC propagation in a conventional duplex stainless steel, which was prepared using an ingot making process. In this figure, the thick line indicates the path along which the SCC propagates. It is apparent that the SCC resistance of a duplex stainless steel is determined by that of the ferritic phase contained therein. Such a ferritic phase unavoidably contains about 4% of nickel due to redistribution between the ferritic phase and the austenitic phase during solidification. Therefore, duplex stainless steel exhibits SCC resistance inferior to that of Ni-free ferritic stainless steel.
Conventional duplex stainless steel must contain 4-8% by weight of nickel so as to make a dual phase, and even the ferritic phase thereof contains 3-6% by weight of nickel due to redistribution between the ferritic phase and the austenitic phase. Therefore, the resulting steel exhibits less SCC resistance than does Ni-free ferritic stainless steel.
It is desirable to provide less expensive pipes for use in the construction of heat-exchanging tubes and piping to which sea-water or industrial water is fed. Since heat exchanging tubes are frequently used under extremely severe conditions, they must exhibit high resistance to SCC. For this purpose, a variety of steel alloys have been proposed, and these are shaped into seamless steel pipes. As is well known in the art, stainless steel has been widely used to make seamless boiler tubes or piping due to its good resistance to corrosion as well as good mechanical properties.
It has been known in the art that a powder metallurgical process can be applied to provide a sintered seamless pipe. However, it has not yet been known in the art to mix different types of stainless steel powder in order to improve corrosion resistance, especially the resistance to SCC. Shodoshima et al. described the high temperature behavior of a sintered product of mixed powders of AISI 430L type powder and AISI 304L type powder ("TETSU-TO-HAGANE", Vol. 76, No. 13, 1981, S1160), stating that the sintered product after cold rolling has a density which is 96.4% of that of the product manufactured through an ingot making process. They also state that the tensile strength and elongation at high temperatures (700.degree.-900.degree. C., for example) are on the average the same as those of a sintered product of one type of powder, i.e. a mono-powder. However, they made no suggestions as to synergistic effects on improvement in chemical or physical properties, especially on improvement in the resistance to SCC, which is achieved by preventing the diffusion of Ni from an austenitic stainless steel powder and/or dual phase stainless steel powder and/or triple phase stainless steel powder to a ferritic stainless steel powder during sintering.